my impression was very much that the evidence points towards the Soviets

This is actually a common misconception that is maintained and spread for obvious reasons. The “official” version relies on everyone already accepting it as true and proceeding from there.

The reality is that there is EXACTLY ZERO evidence that Soviets did it and all evidence is compatible or strongly suggests German guilt. All the “evidence” the proponents of Soviet guilt cite is either circular citations or proven forgeries.

You can read an exhaustive analysis of all relevant evidence here:

https://archive.org/details/the-mystery-of-the-katyn-massacre-the-evidence-the-solution-grover-furr-2018

The book is structured such that Chapter 1 already presents the full argument, then the next few chapters scrutinize the unimpeachable evidence in detail, and then the next few present some secondary and confirmatory evidence.